JPH10165819A - Catalyst for cleaning of exhaust gas and its use method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve NOx cleaning performance in a lean atmosphere by preparing such a catalyst that contains alumina or zeolite which carries one or more kinds of noble metals selected from platinum, palladium, rhodium and iridium and contains a multiple oxide expressed by the formula, and by depositing platinum, on zeolite. SOLUTION: As for the noble metal in the catalyst for cleaning of exhaust gas, at least one kind selected from platinum, palladium, rhodium and iridium is used. As for the base body to carry the noble metal, a heat-resistant inorg. material having a large specific surface area is preferable, especially so as to maintain the dispersibility of the noble metal after the catalyst is used for a long time, and alumina or zeolite is preferably used. The multiple oxide included in the catalyst is expressed by (La1-x Ax )1-αBO1-δ , wherein x, α and δsatisfy 0<x<1, 0<α<0.2, 0ηδ<1, and A is barium and/or potassium, B is least one element selected from iron, cobalt, nickel and manganese, and this multiple oxide contains lanthanum, potassium, barium, iron etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification system for purifying hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine of an automobile or the like. The present invention relates to a catalyst and a method of using the same, and more particularly to an exhaust gas purifying catalyst having excellent NOx purification performance in an oxygen-excess atmosphere and a method of using the same.

[0002]

2. Description of the Related Art In recent years, in view of the problem of depletion of petroleum resources and the problem of global warming, realization of low fuel consumption vehicles is expected, and development of lean burn vehicles is particularly desired for gasoline vehicles. . In lean-burn vehicles, the exhaust gas atmosphere during lean-burn operation is based on the theoretical air-fuel condition
An oxygen-excess atmosphere (hereinafter, referred to as a “lean atmosphere”) as compared to “stoichiometric state”. When a conventional three-way catalyst is applied in a lean atmosphere, there has been a problem that the effect of excessive oxygen makes the NOx purification action insufficient. For this reason, NOx
There has been a demand for the development of a catalyst that can purify methane.

Conventionally, NOx in a lean atmosphere has been
Various catalysts for improving the purification performance have been proposed, and are roughly classified into two types. One is to oxidize and purify NOx using HC in the exhaust gas as a reducing agent, and the other is to absorb NOx in a lean atmosphere and emit NOx in a stoichiometric or fuel-rich (rich) atmosphere. It purifies.

[0004] As a representative of the former, for example, JP-A-63-100919 discloses a catalyst in which copper (Cu) is supported on zeolite.

On the other hand, as a representative of the latter, for example, JP-A-5-168860 discloses a catalyst in which lanthanum or the like is supported on platinum (Pt) and lanthanum is used as a NOx absorbent.

[0006]

However, the above-mentioned conventional NOx purification catalyst (for example, a Cu-supported zeolite catalyst)
And NOx absorption catalyst (for example, Pt-lanthanum catalyst)
Due to its characteristics, the former cannot achieve a sufficient purifying action if the HC / NOx ratio in the exhaust gas is small, and the latter does not reach the saturation when the steady running in a lean atmosphere causes the NOx absorption amount to reach saturation. NOx purification performance is insufficient, the performance after durability is not sufficient, and NOx cannot be purified under a wide range of operating conditions.

Accordingly, it is an object of the present invention to improve the NOx purifying performance under a lean atmosphere, which has not exhibited sufficient activity with a conventional catalyst, and to function as a three-way catalyst. It is an object of the present invention to provide an exhaust gas purifying catalyst which can be sufficiently developed.

It is another object of the present invention to provide a method of using an exhaust gas purifying catalyst which can exhibit the NOx purifying action of the exhaust gas purifying catalyst of the present invention particularly effectively.

[0009]

According to a first aspect of the present invention, there is provided an exhaust gas purifying catalyst which carries at least one noble metal selected from the group consisting of platinum, palladium, rhodium and iridium for each of the selected noble metal elements. Alumina or zeolite and the following general formula

(Equation 2) And platinum is supported on zeolite.

Further, in order to further enhance the above-mentioned action of the exhaust gas purifying catalyst according to the first aspect, suppress alloying of noble metals, and obtain high thermal durability, the exhaust gas purifying catalyst according to the second aspect. The catalyst for use is characterized in that at least one noble metal selected from the group consisting of palladium, rhodium and iridium is supported on alumina for each selected noble metal element.

In order to efficiently realize the NOx absorbing action and the purifying action of the exhaust gas purifying catalyst according to the first or second aspect, the exhaust gas purifying catalyst according to the third aspect has at least two catalyst layers. And the lower layer contains the composite oxide.

Further, in order to further enhance the NOx purifying action of the exhaust gas purifying catalyst according to the third aspect, the exhaust gas purifying catalyst according to the fourth aspect contains a platinum-supported zeolite in an upper layer. .

Further, in order to further enhance the above-mentioned action of the exhaust gas purifying catalyst according to the fourth aspect and to maintain a higher activity even after durability, the exhaust gas catalyst according to the fifth aspect includes rhodium and And / or containing iridium-supported alumina.

Further, in order to exhibit an effective NOx absorption / release cycle of the exhaust gas purifying catalyst of the present invention,
According to a sixth aspect of the present invention, there is provided a method of using the exhaust gas purifying catalyst according to the present invention, wherein the exhaust gas purifying catalyst is used in a lean burn engine vehicle in which the air-fuel ratio repeats stoichiometry and a range of 15 to 50. Features.

As the noble metal in the exhaust gas purifying catalyst of the present invention, at least one selected from the group consisting of platinum, palladium, rhodium and iridium is used. For example, various combinations such as Pt and Rh, Pd and Rh, and Pd alone are possible. The content of the noble metal is not particularly limited as long as the NOx absorption capacity and the three-way catalyst performance are sufficiently obtained, but if the content is less than 0.1 g, sufficient three-way performance cannot be obtained,
Since no significant improvement in characteristics is observed even when the amount is more than 10 g, 0.1 L per 1 L of the exhaust gas purifying catalyst of the present invention is used.
-10 g are preferred.

A heat-resistant inorganic material having a large specific surface area is suitable for the base material for supporting the noble metal, in order to secure the dispersibility of the noble metal, particularly, the dispersibility of the noble metal after durability, and alumina or zeolite is particularly preferable. Activated alumina is particularly desirable as the alumina, and pentasil-type zeolite, Y-type zeolite, monodenite, ferrierite and the like are desirable as the zeolite. Alumina or zeolite to which a rare earth element, zirconia, or the like is added to increase the heat resistant specific surface area may be used. If the amount of alumina or zeolite used is less than 50 g per 1 L of the catalyst of the present invention, sufficient dispersibility of the noble metal cannot be obtained, and if it is used more than 300 g, the performance is deteriorated.

The composite oxide contained in the exhaust gas purifying catalyst of the present invention has the following general formula:

(Equation 3) It is represented by

The composite oxide used in the exhaust gas purifying catalyst of the present invention contains a rare earth metal, an alkali metal and / or an alkaline earth metal, and at least one transition metal. Lanthanum is preferably used as the rare earth metal, potassium is used as the alkali metal, barium is used as the alkaline earth metal, and iron, cobalt, nickel and manganese can be suitably used as the transition metal.

A composite oxide such as the above-described perovskite-type oxide has characteristics of causing oxygen deficiency, easily adsorbing NOx through the generated oxygen deficiency, and absorbing NOx in a lean atmosphere. Utilization makes it possible to improve NOx purification performance.

Further, the perovskite-type oxide may cause a solid-phase reaction with the alumina-based oxide in the catalyst composition to deactivate the activity. In order to suppress this, there are a method of pre-coating lanthanum or the like on the alumina-based oxide and a method of using a material having low reactivity with perovskite such as zirconia. On the other hand, by slightly losing the A site of the perovskite-type oxide from the stoichiometric ratio as in the present invention, the solid-phase reaction between the perovskite-type oxide and another oxide (alumina or the like) in contact therewith To suppress
It has become possible to improve the thermal stability.

The substitution amount of the A site is 0 <X <1 and is not particularly limited. However, in order to obtain a sufficient NOx absorption capacity, it is particularly preferable that 0.2 ≦ X <1.

When the value of α exceeds 0.2, a single-phase perovskite structure is not formed, so that 0 <α <0.2 is preferable. The value of δ is 0 ≦ δ <1, but from the viewpoint of stability of the crystal structure, it is particularly preferable that 0 ≦ δ ≦ 0.5.

The amount of the composite oxide functioning as a NOx absorbing material is not particularly limited as long as it exhibits an NOx absorbing effect, but if it is less than 10 g, a sufficient NOx absorbing ability cannot be obtained, and if it is more than 200 g, Since no significant improvement in properties is observed even when used, 10 L per 1 L of the catalyst of the present invention is used.
~ 200 g is preferred.

The composite oxide used in the present invention, particularly the partially substituted perovskite oxide, exhibits the ability to absorb NOx in a lean atmosphere together with the partial substitution amount. It is considered that NOx is oxidized to NO ₂ on the composite oxide and is absorbed in the vicinity of barium and / or potassium on the surface of the composite oxide in the form of a nitric acid group or a state close thereto. Therefore, the composition of the composite oxide for effectively absorbing NOx under a lean atmosphere is as follows:
Nitrate containing barium and / or potassium may be easily manufactured, and further, it is important to contain a transition metal element capable of oxidizing NOx to NO _2.

Each of the constituent elements of the composite oxide exerts the above-mentioned effects to the maximum when all of these contained in the catalyst are complexed, but at least a part of the complex oxide forms a complex. Even if it is possible, the above effect can be sufficiently obtained. Each constituent element of the composite oxide can exist as a composite oxide without being separated as a separate oxide even after thermal endurance, and this can be confirmed by, for example, X-ray diffraction measurement.

In the exhaust gas purifying catalyst of the present invention, it is possible to obtain a NOx purifying action which cannot be obtained alone by coexisting the noble metal-supported alumina or zeolite and the composite oxide. That is, when the exhaust gas atmosphere becomes lean, high NOx purification performance can be obtained by the NOx absorbing action of the composite oxide in the exhaust gas purification catalyst of the present invention. The NOx absorbing effect of the composite oxide is to obtain an excellent NOx purification performance that cannot be obtained by simply mixing the individual components constituting the composite oxide.

When the atmosphere of the exhaust gas changes from lean to stoichiometric, NOx is released from the composite oxide.
NOx purification can be effectively carried out by repeating a cycle of purification using noble metal-supported alumina or zeolite powder. This is because the porous structure of alumina and zeolite affects gas diffusion, and provides a suitable gas atmosphere to the noble metal supported on alumina or zeolite.

Further, the catalyst of the present invention has a high NOx absorbing effect even after heat endurance. This is because the composite oxide has a perovskite type structure having a small proportion of A site, and has a high NOx absorption effect with other components (for example, alumina). This is because the solid-phase reaction was avoided.

By supporting platinum on zeolite, the action of purifying NOx released from the absorbent in the medium is enhanced.

The noble metal selected from the group consisting of palladium, rhodium and iridium is preferably supported on alumina. Thereby, the exhaust gas purifying catalyst according to the second aspect is obtained. A base material for supporting a noble metal selected from the group consisting of palladium, rhodium and iridium is made of a heat-resistant inorganic material having a large specific surface area, in order to ensure the dispersibility of the noble metal, particularly the durability of the noble metal after durability. Alumina is particularly suitable, and activated alumina to which a rare earth element, zirconia, or the like is added in order to increase the heat resistant specific surface area may be used. By supporting platinum and palladium, rhodium, and iridium on separate substrates, alloying of noble metals can be suppressed, and high post-durability performance can be obtained.

Preferably, at least two catalyst layers are provided, and the composite oxide is contained in the lower layer to obtain the exhaust gas purifying catalyst according to claim 3, and the platinum-supported zeolite is further contained in the upper layer. Thus, the exhaust gas purifying catalyst according to claim 4 is obtained.

With such a configuration, a high NO
The x absorbing action and the purifying action of NOx released from the absorbing material can be compatible. This is because the HC is once purified in the upper layer, so that the exhaust gas with less HC reaches the lower layer, and as a result, N
This is because the Ox absorption effect is enhanced, and in particular, since the upper layer containing the platinum-supported zeolite can be brought into contact with the exhaust gas having a high HC concentration, the NOx released from the absorbent can be efficiently purified.

Further, with such a structure, the NOx reducing action in the oxygen-excess atmosphere which the noble metal originally has can be efficiently obtained, and the NOx purification performance is further improved. Since the NOx reduction reaction is a mechanism for reducing NOx by HC, it is preferable that the HC concentration is high.
Since the platinum-carrying zeolite that adsorbs C is disposed on the contact surface layer, a large amount of HC can be supplied to the noble metal, so that NOx reduction can be performed more efficiently, and the NOx purification performance can be significantly improved.

Preferably, the upper layer further contains rhodium and / or iridium-supported alumina.
An exhaust gas purifying catalyst as described is obtained. Thereby, it has high activity with respect to the above-mentioned action even after heat endurance.

The exhaust gas purifying catalyst of the present invention can be used especially for a lean burn engine vehicle in which the air-fuel ratio repeats stoichiometry and a range of 15 to 50. With such a usage method, NOx
The absorption / release cycle is established very effectively, and particularly efficient NOx purification becomes possible.

As the noble metal raw material compound for preparing the catalyst used in the present invention, nitrates, carbonates, ammonium salts, acetates, halides, oxides and the like can be used in combination. In particular, water-soluble salts are used. Is preferable from the viewpoint of improving the catalyst performance. The method for supporting the noble metal is not limited to a special method, and various methods such as a known evaporation and drying method, a precipitation method, an impregnation method, and an ion exchange method can be used as long as there is no significant uneven distribution of components. In particular, for the support on zeolite, the ion exchange method is desirable from the viewpoint of ensuring the dispersibility of the noble metal.

The composite oxide used in the present invention is prepared by mixing nitrates, acetates, carbonates and the like of the respective constituent elements of the composite oxide in a desired composition ratio of the composite oxide, calcining the mixture, and pulverizing the mixture. After the solid-phase reaction to be subjected to heat treatment and firing, nitrate, acetate or carbonate, hydrochloride, citrate, etc. of each constituent element of the composite oxide are mixed in a desired composition ratio of the composite oxide, and then dissolved in water. If necessary, an alkaline solution such as NH ₄ OH or NH ₃ CO ₃ may be added dropwise to form a precipitate, which can be prepared by a known method such as a coprecipitation method in which the precipitate is filtered, dried, and fired. By such a method, at least a part of each component constituting the composite oxide can be composited.

The raw material for preparing the catalyst used in the present invention may contain a trace amount of impurities as long as the above-mentioned action is not impaired. For example, strontium contained in barium, lanthanum, neodymium contained in cerium, And samarium.

For use in the present invention thus obtained,
The noble metal-supported alumina or zeolite, the composite oxide and the noble metal-supported alumina are each pulverized to form a slurry, coated on a catalyst support, and calcined at a temperature of 400 to 900 ° C., whereby the exhaust gas purifying catalyst of the present invention is obtained. Obtainable.

The catalyst carrier can be appropriately selected from known catalyst carriers and used, and examples thereof include a honeycomb carrier and a metal carrier having a monolith structure made of a refractory material. Although the shape of the catalyst carrier is not particularly limited, it is usually preferable to use a honeycomb shape. As the honeycomb material, cordierite materials such as ceramics are generally used,
It is also possible to use a honeycomb made of a metal material such as a ferritic stainless steel, and further, the catalyst powder itself may be formed into a honeycomb shape. By making the shape of the catalyst into a honeycomb shape, the area of the catalyst and the exhaust gas becomes large, and the pressure loss is suppressed, which is extremely advantageous when the catalyst is used for an automobile or the like.

Since the catalyst used in the present invention also needs to function as a three-way catalyst at the time of stoichiometry, additives conventionally used in three-way catalysts may be further added. Ceria and barium, which alleviates HC adsorption and poisoning to noble metals, and zirconia, which contributes to improving the heat resistance of Rh.

[0042]

The present invention will be described below with reference to the following examples and comparative examples. Example 1 Activated alumina powder was impregnated with a rhodium nitrate solution, dried and calcined at 400 ° C. for 1 hour to obtain rhodium-supported activated alumina powder (powder A). The rhodium concentration of this powder A was 2.0% by weight.

The activated alumina powder was impregnated with an aqueous solution of palladium nitrate, dried and calcined at 400 ° C. for 1 hour to obtain a palladium-supported activated alumina powder (powder B). The Pd concentration of this powder B was 5.0% by weight.

The H-type ZSM-5 powder was impregnated with an aqueous solution of dinitrodiammine platinum, dried and calcined at 400 ° C. for 1 hour to obtain a platinum-supported zeolite powder (powder C). The platinum concentration of this powder C was 5.0% by weight.

Citric acid was added to a mixture of lanthanum carbonate, barium carbonate, and cobalt carbonate, dried and calcined at 700 ° C. to obtain a powder D. This powder D had a metal atom ratio of lanthanum / barium / cobalt = 4/5/10.

108 g of the powder A and 21 g of the powder B
2 g, powder 189 g, powder D 225 g,
166 g of activated alumina and 900 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid was added to a cordierite monolithic carrier (1.0 L, 4 L).
After drying at 130 ° C. and removing excess slurry in the cells with an air stream,
Fired for hours. This operation was performed twice, and the coat layer weight 20
0 g / L of the exhaust gas purifying catalyst of the present invention was obtained.

Example 2 Activated alumina powder was impregnated with an aqueous solution of dinitrodiammine platinum, dried and calcined at 400 ° C. for 1 hour to obtain a platinum-supported activated alumina powder (powder E). The platinum concentration of this powder E was 5.0% by weight. Powder B of Example 1 was replaced with Powder E
In the same manner as in Example 1 except that the catalyst was replaced with, an exhaust gas purifying catalyst of the present invention was obtained.

Example 3 Activated alumina powder was impregnated with an iridium chloride aqueous solution, dried and calcined at 600 ° C. for 1 hour to obtain iridium-supported activated alumina powder (powder F). The iridium concentration of this powder F was 2.0% by weight.

108 g of powder A, 212 g of powder B, 189 g of powder C and 225 g of powder D obtained in Example 1
81 g of the powder F, 86 g of activated alumina, and 9 parts of water
00g was charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid was adhered to a cordierite-based monolithic carrier (1.0 L, 400 cells), excess slurry in the cells was removed by an air stream, dried at 130 ° C., and fired at 400 ° C. for 1 hour. This operation was performed twice to obtain an exhaust gas purifying catalyst of the present invention having a coat layer weight of 200 g / L-carrier.

Example 4 108 g of the powder A obtained in Example 1 and 189 of the powder C were obtained.
g, 225 g of powder D, 212 g of powder E obtained in Example 2, 81 g of powder F obtained in Example 3, 86 g of activated alumina, and 900 g of water were charged into a magnetic ball mill and mixed and pulverized. Thus, a slurry liquid was obtained. This slurry liquid was added to a cordierite monolithic carrier (1.0 L, 400 L).
After drying at 130 ° C., the mixture was baked at 400 ° C. for 1 hour. This operation was performed twice, and the coat layer was 200 g / L.
-A catalyst for purifying exhaust gas of the present invention as a carrier was obtained.

Example 5 The same procedure as in Example 1 was repeated, except that the powder D in Example 1 was replaced by a powder having a metal atom ratio of lanthanum / barium / cobalt = 2/7/10. An exhaust gas purifying catalyst was obtained.

Example 6 The procedure of Example 1 was repeated, except that the powder D in Example 1 was replaced by a powder having a metal atom ratio of lanthanum / barium / cobalt = 7/2/10. An exhaust gas purifying catalyst was obtained.

Example 7 Except that iron in the powder D in Example 1 was changed to iron.
An exhaust gas purifying catalyst of the present invention was obtained in the same manner as in Example 1.

Example 8 An exhaust gas purifying catalyst of the present invention was obtained in the same manner as in Example 1 except that the cobalt of the powder D in Example 1 was changed to nickel.

Example 9 An exhaust gas purifying catalyst of the present invention was obtained in the same manner as in Example 1 except that manganese was used as the cobalt for the powder E in Example 1.

Example 10 144 g of powder A obtained in Example 1 and 282 of powder B were obtained.
g, powder D 300 g, activated alumina powder 174 g,
900 g of water is put into a magnetic ball mill, mixed and pulverized, and the slurry liquid is applied to a cordierite-based monolithic carrier (1.0%).
L, 400 cells), and the excess slurry in the cells was removed by an air stream and dried at 130 ° C.
It baked at 1 degreeC for 1 hour, and obtained the coat layer 150g / L-support | carrier.

378 g of the powder C obtained in Example 1, 522 g of the activated alumina powder, and 900 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid is adhered to the above carrier a, excess slurry in the cells is removed by an air stream, and the slurry is dried at 130 ° C.
It was calcined at 0 ° C. for 1 hour to obtain an exhaust gas purifying catalyst of the present invention having a coat layer weight of 250 g / L-carrier.

Example 11 282 g of powder B obtained in Example 1 and 300 g of powder D were obtained.
g, activated alumina 318 g and water 900 g were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid was added to a cordierite monolithic carrier (1.0
L, 400 cells), and the excess slurry in the cells was removed by an air stream and dried at 130 ° C.
It baked at 1 degreeC for 1 hour, and obtained the coat layer 150g / L-carrier | rotor.

The powder A obtained in Example 1 was mixed with 216
g, powder 378 g, activated alumina 306 g, water 9
00g was charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid is adhered to the carrier b,
The excess slurry in the cell is removed by air flow to remove 130
After drying at 400 ° C., the mixture was calcined at 400 ° C. for 1 hour to obtain an exhaust gas purifying catalyst of the present invention having a coat layer weight of 250 g / L-carrier.

Comparative Example 1 An exhaust gas purifying catalyst of the present invention was obtained in the same manner as in Example 1 except that the powder C in Example 1 was changed to activated alumina.

Comparative Example 2 An exhaust gas purifying catalyst of the present invention was obtained in the same manner as in Example 1 except that the powder obtained by removing lanthanum from the powder D of Example 1 was used.

Comparative Example 3 An exhaust gas purifying catalyst of the present invention was obtained in the same manner as in Example 1 except that the powder obtained by removing barium from the powder D of Example 1 was used.

Comparative Example 4 An exhaust gas purifying catalyst of the present invention was obtained in the same manner as in Example 1, except that the powder in Example 1 except for cobalt in Powder D was used.

Table 1 shows the catalyst compositions of the exhaust gas purifying catalysts obtained in Examples 1 to 11 and Comparative Examples 1 to 4.

[0065]

[Table 1]

Test Examples The exhaust gas purifying catalysts obtained in Examples 1 to 11 and Comparative Examples 1 to 4 were evaluated for the initial and endurance catalytic activities under the following conditions. For the activity evaluation, an automatic evaluation device using a model gas simulating the exhaust gas of an automobile was used.

Endurance conditions A catalyst was mounted in an exhaust system of 4400 cc engine, and operation was performed at a catalyst inlet temperature of 700 ° C. for 50 hours to endurance.

Evaluation Conditions The catalyst activity was evaluated by mounting each catalyst in an exhaust system of a 2000 cc engine with an A / F = 14.6 (stoichiometric state) for 30 seconds, and then A / F = 22 (lean atmosphere).
And the average conversion rate was measured for each cycle, and the average conversion rate when A / F = 14.6 (stoichiometric state) and the average when A / F = 22 (lean atmosphere). The conversion rate was averaged to obtain a total conversion rate. This evaluation was performed at the initial stage and after the endurance test, and the catalytic activity evaluation value was determined by the following equation.

[0069]

(Equation 4)

Table 2 shows the catalytic activity evaluation results obtained as the total conversion. The catalytic activity of the example was higher than that of the comparative example, and the effect of the present invention described later could be confirmed.

[0071]

[Table 2]

[0072]

The exhaust gas purifying catalyst according to claim 1 is
The conventional catalyst can improve the NOx purification performance under a lean atmosphere, which did not show sufficient activity, and can sufficiently exhibit the function as a three-way catalyst. Can be shown.

The exhaust gas purifying catalyst according to claim 2 further secures sufficient noble metal dispersibility in addition to the above effects,
High heat durability can be obtained by suppressing alloying of noble metals.

In the exhaust gas purifying catalyst according to the third aspect, in addition to the above effects, the NOx absorbing function and the NOx purifying function can be effectively realized, and the NOx purifying can be made more efficient.

The exhaust gas purifying catalyst according to the fourth aspect can further enhance the NOx purifying action in addition to the above effects.

The exhaust gas purifying catalyst according to claim 5 can maintain higher activity even after heat endurance, in addition to the above effects.

The exhaust gas purifying catalyst according to the sixth aspect is capable of exhibiting the effective NOx absorption / release cycle of the exhaust gas purifying catalyst of the present invention particularly efficiently.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B01J 23/78 ZAB B01J 29/068 23/89 ZAB 29/48 ZABA 29/064 ZAB B01D 53/36 ZAB 29/068 102A 29 / 48 ZAB 102H 104A B01J 23/64 104A

Claims (6)

[Claims] 1. An alumina or zeolite carrying at least one noble metal selected from the group consisting of platinum, palladium, rhodium and iridium for each selected noble metal element, and the following general formula: An exhaust gas purifying catalyst comprising a composite oxide represented by the formula: and platinum supported on zeolite. 2. The exhaust gas purification according to claim 1, wherein at least one noble metal selected from the group consisting of palladium, rhodium and iridium is carried on alumina for each of the noble metal elements selected. Catalyst. 3. The exhaust gas purifying catalyst according to claim 1, wherein at least two catalyst layers are provided, and the lower layer contains the composite oxide. 4. The exhaust gas purifying catalyst according to claim 3, wherein the upper layer contains a platinum-supported zeolite. 5. The exhaust gas purifying catalyst according to claim 4, wherein the upper layer contains rhodium and / or iridium-supported alumina. 6. The exhaust gas purifying catalyst according to claim 1, wherein the air-fuel ratio is 15% or less.
A method for using an exhaust gas purifying catalyst, wherein the method is used for a lean burn engine vehicle that repeats a range from 50 to 50.